This invention relates to roofing structures, and more particularly to a system and method for enhancing the bond of roofing membranes to lightweight insulating concrete.
Roofing structures often include a roofing deck followed by one or more layers of lightweight insulating concrete and a roofing membrane. Roofing membranes typically comprise multiple plies of bituminous felt material that are sealed together and to the concrete layer with flood pourings of hot roofing asphalt. It has been found, however, that full adhesion of the roofing membrane to the concrete layer is prevented by imperfections on the surface of the concrete layer, such as surface dusting, soft spots, and/or low spots. Consequently, isolated air pockets may form between the concrete layer and the roofing membrane, thereby decreasing the strength of the interfacial bond formed between the two layers. In addition, moisture that may be trapped in the pockets can cause the formation of bubbles and subsequent leaks in the roofing membrane, especially on hot days where expansion of the water vapor occurs. Moreover, the water-to-cement ratio for lightweight insulating concrete is typically several times that of conventional structural cement in order to render the material sufficiently fluid for placement on the roof platform. The bituminous plies are often installed before the concrete is completely cured and dried, due to the uneconomical delay in the curing and drying process. Consequently, air and moisture can easily become trapped within pockets beneath the membrane layer. In addition, external sources of moisture, such as humidity or rain, may also cause the build-up of moisture beneath the waterproofing layer.
In an effort to overcome these problems, the prior art has proposed various solutions that provide adequate ventilation between the concrete and membrane layers. However, the predictability of the adhesion strength between the layers and/or the cost of such solutions are often compromised. By way of example, U.S. Pat. No. 4,803,111 to Mansell discloses a membrane roofing system wherein a thin, perforated non-bituminous sheet of underlay material is installed over a substrate. An adhesive is applied over the underlay material and onto the substrate at areas exposed by the perforations. A membrane layer is then placed over the underlay material so that vapor trapped between the membrane and substrate can disperse through the areas of non-adhesion between the membrane and substrate. However, this system suffers from unpredictable adhesion strength due to the aforementioned surface defects of the concrete layer.
Thus, the interfacial bonds between the concrete and membrane layers of the foregoing solutions may not be as strong as desired, subjecting the roofing system to premature failure due to a phenomenon known as xe2x80x9cwind uplift.xe2x80x9d According to this phenomenon, the lateral movement of air (wind) over the top surface of the roofing system causes a reduction in air pressure above the roof, similar to air pressure reduction which occurs over an airplane wing in flight. The reduced air pressure above the roofing system imparts forces orthogonal to the plane of the roofing system, resulting in xe2x80x9cupliftxe2x80x9d of the roof assembly. These forces tend to pull apart the various layers of the composite roofing systems described above, thereby inducing failure of the roofing system.
Wind, downward load, seismic activity and other phenomena may also impart lateral forces along the face of the top layer of the roofing system, and these lateral forces are often transmitted to the lower insulating layers. Such lateral forces, also designated as horizontal shear forces, may contribute to wind uplift failure, particularly when acting in conjunction with transverse forces associated with wind uplift. They may also decrease the downward load capacity of the roofing system. Because of their susceptibility to this type of failure, such composite roofing systems may fail to meet increasingly stringent building codes, insurance requirements, and other regulatory requirements, particularly in geographic regions where strong winds are common.
In view of the above problems associated with adhesively secured roof membrane systems, the preferred method for the past 30 years has been to mechanically fasten the first ply of the membrane directly to the lightweight insulating concrete. Although this method allows vapor to flow freely between the membrane and concrete layers and ensures a predictable bond between the layers, the cost of mechanical fasteners as well as the requisite skilled labor and installation time are disadvantages which heretofore have been difficult to overcome.
Accordingly, it is desirable to provide a system and method for attaching a waterproof membrane layer to a lightweight insulating concrete layer that permits vapor flow and ensures a predictable bond between the layers without the use of mechanical fasteners.
In accordance with one aspect of the invention, a roof structure comprises a lightweight insulating concrete layer and a waterproof membrane layer overlying the concrete layer. The waterproof membrane layer has a first ply of waterproof material and a first adhesive layer on the first ply. The roof structure also comprises a plurality of adhesive pellets at least partially embedded in the concrete layer. At least some of the adhesive pellets are in contact with the first adhesive layer of the first ply to thereby adhesively bond and mechanically connect the waterproof membrane layer to the lightweight insulating concrete layer.
In accordance with a further aspect of the invention, a roof structure comprises a lightweight insulating concrete layer with a mixture of Portland cement and at least one aggregate material taken from the group consisting of vermiculite, perlite or pregenerated cellular foam. A waterproof membrane layer overlies the concrete layer. The waterproof membrane layer comprises a first ply of waterproof material with a first heat responsive adhesive layer and a second ply of waterproof material with a second heat responsive adhesive layer bonded to the first ply to thereby secure the first and second plies together. The first adhesive layer has a plurality of spaced adhesive elements on an underside of the first ply, with spaces between the adhesive elements forming vent pathways for vapor movement and vapor pressure relief between the first ply and the concrete layer. A plurality of heat responsive adhesive pellets are at least partially embedded in the concrete layer. The adhesive pellets are melted together with the spaced adhesive elements of the first adhesive layer to thereby adhesively bond and mechanically connect the waterproof membrane layer to the lightweight insulating concrete layer.
In accordance with an even further aspect of the invention, a method of constructing a roof structure comprises forming a lightweight insulating concrete layer; distributing a plurality of adhesive pellets over the concrete layer while the concrete layer is in the plastic state so that the adhesive pellets are at least partially embedded in the concrete layer; at least partially curing the concrete layer so that the adhesive pellets are mechanically secured to the concrete layer; providing a waterproof membrane layer having a first ply of waterproof material and a first adhesive layer on a lower surface of the first ply; and engaging at least some of the adhesive pellets with the first adhesive layer to thereby adhesively bond and mechanically connect the first ply to the concrete layer.